1. Technical Field
The present disclosure relates generally to the treatment of respiratory and cardiovascular disorders with a mechanical ventilator, and more particularly, to single respiratory cycle patient ventilation flow limitation detection.
2. Related Art
The respiration system of the human body provides needed oxygen intake, oxygen/carbon dioxide exchange, and carbon dioxide expulsion functions, each of which involves the lungs. In this regard, the lungs function as a gas-exchanging organ in which inhaled oxygen is passed to the blood, and collected carbon dioxide is passed from the blood to the air. Additionally, the lungs function as a respiratory pump that transports oxygen-rich air into the lungs, and the carbon dioxide-rich air out of the lung. The breathing center in the brain, central and peripheral nerves, the osseous thorax and the breathing musculature as well as free, stable respiratory paths are necessary for a correct functioning of the respiratory pump.
There are a variety of conditions that adversely affect the respiratory function of a person, particularly during sleep. Among these is apnea, where airflow to the lungs is interrupted and the normal respiratory cycle is broken. The stopped airflow or apnea may result from a failure of the basic neurological controls over breathing, with no breathing effort being expended. This type of apnea is known as central sleep apnea (CSA). Alternatively, the apnea may result from a constriction in the upper airway that also interrupts normal respiration, but while the patient exerts breathing effort. This type is known as obstructive sleep apnea (OSA). Where the patient's breathing efforts overcome the obstruction yet there is a significant reduction in airflow, there is understood to be hypopnea. There are repetitive pauses in breathing that may extend in duration up to half a minute. These conditions, at the very least, result in disruptions to sleep cycles because the patient is aroused to a waking state in an attempt to achieve proper respiration, leading to daytime drowsiness and fatigue as a consequence of reduced blood oxygen saturation and/or increased blood carbon dioxide concentration. In more severe cases, blood oxygen saturation may be so reduced (a condition referred to as hypoxemia), or the blood carbon dioxide concentration may be so high (a condition referred to as hypercapnia) that morbidity may be result.
In order to retain the patient's airway and ensure normal, uninterrupted breathing during sleep, continuous positive airway pressure (CPAP) therapy may be prescribed. Generally, CPAP involves the application of positive pressure to open the patient's airway to prevent its collapse, as would otherwise occur during apnea. In a basic implementation, CPAP therapy applies a constant pressure that is not tied to the patient's normal breathing cycle. The positive airway pressure is desired in the inspiratory phase when the pressure differences between the lungs and the nose contribute to the collapse of the intermediate airway. Such implementations were typically uncomfortable for the patient as there were differing augmentation needs depending on the degree of obstruction, and the relative point within the breathing cycle. Accordingly, CPAP systems with varied pressure augmentation based on the detection of full or partial obstruction of the airway were developed.
Existing flow limitation detection techniques are understood to be based upon the understanding that partial airway obstructions as with OSA result in mid-inspiratory flow limitation. One technique involves a calculation of the index of a partial obstruction through a shape factor, as set forth in U.S. Pat. No. 6,029,665 as well as U.S. Pat. No. 6,138,675 both to Berthon-Jones. Another technique involves a calculation of the degree of flow limitation defined as a series of shape detection factors, including a sinusoidal index, a flatness index, respiratory effort index, and relative flow index.
All of these conventional methods, however, are deficient since multiple indices must be compared to a predefined threshold in order to evaluate whether a flow limitation corresponding to an obstructed respiration condition exists. Accordingly, there is a need in the art for an improved method for single respiratory cycle patient ventilation flow limitation detection.